1. Field of the Invention
The invention relates to a method for detecting objects and, more particularly, to a method for detecting an object with a frequency modulated continuous wave (FMCW) ranging system.
2. Description of the Related Art
Conventional FMCW ranging systems may be sound or microwave-based, and are used for performing distance or level measurements in industrial process control, factory automation or automotive applications.
Here, a transmitted signal is modulated to periodically sweep over a predetermined frequency range. A received signal, which comprises echo signal portions from the object of interest and other obstacles, is mixed with the transmitted signal and the result is analyzed by, e.g., a Fast Fourier Transform (FFT) to obtain a frequency spectrum in which the echoes appear as peaks. The echoes or peaks in the frequency spectrum (i.e., the echo profile) may be separate or may overlap.
When, for example, measuring the level of material within a closed vessel, the echo from the material is often distorted by echoes from other scatterers, such as obstacles or welded seams. Here, the distortion in the echo introduces error in the level being reported when the echo from the material is close to an echo from an obstacle. It is known that the echoes from scatterers often overlap the main echo, and that there is no clear discrimination between them. This overlapping of the scatterers and the main echo is an intrinsic limitation of FMCW ranging systems.
Typically, radar devices operating in enclosed vessels have a wide bandwidth to achieve high resolution, i.e., the ability to resolve two targets. To operate in open air, however, the bandwidth is governed by other sets of rules, such as those of the Federal Communications Commission (FCC) in the U.S., and it is generally limited to a much narrower bandwidth. A narrower bandwidth, however, will cause the frequency difference between two targets to be smaller. A narrower bandwidth also means that the peaks on the echo profile will be much wider. The combination of these two factors reduces greatly the ability of the device to operate in open air and still achieve reasonable accuracy or ranging resolution.
The most commonly applied technique to identify an echo of interest relies on the power spectrum density (PSD), usually calculated by an FFT, to estimate the frequency of interest as the frequency corresponding to the largest component of the FFT-PSD. Because of the sampled nature of the FFT-PSD, the largest component of the PSD may not be the peak of the true spectrum or the true peak of interest whose accurate position can then be estimated by using a center of mass algorithm about the largest component in the power spectrum.
An FFT and estimations based on the power spectrum discard a lot of information and limit the separation power of current FMCW devices.